Inspection and reliable verification of printed circuit patterns has been a significant cost factor in the production and manufacture of electronic systems. Such printed circuit patterns contain complex circuit configurations which may be linear or curvi-linear and in which the circuit lines, and the spaces between conductors may be quite small, as in the order of a few mils.
Human visual inspection of such circuitry with required magnification assistance is slow, time consuming, fatiguing for the inspector, often inconsistent, costly, and not reliable. Usually the speed of inspection varies from 0.5 to 2.0 square inches per minute depending upon the inspector and his state of fatigue.
Prior proposed inspection methods have included optical inspection by a "windowing technique" and by a "design rule" technique. Generally in the windowing technique a pixel by pixel comparison to a stored image of the circuitry under test is made through defined rectangular "windows". These windows must be individually manually selected by the operator. The number of windows may be in the hundreds or thousands depending upon the circuitry, and the more complex the circuitry, the more windows are required. Since the "window" areas must be manually defined the time required to "teach" the system, or prepare for the inspection of a new type of circuit, is usually many hours. Such "window" systems are limited to orthogonal arrangements with no curves or diagonal circuit portions. The speed of inspection by "windowing technique" may usually be several seconds per square inch.
The "design rule" method generally involves checking that the conductor line widths meet or exceed a predefined minimum and that spacing between conductors is never less than an allowed minimum. The design rule checking system operates locally, checks only general features which are universal to all acceptable circuit patterns, and detects only flaws which violate the design rule applied. For example, the absence of certain types of conductor segments will not violate a design rule.